billing: Integrate Stripe, using Stripe Checkout.

Stripe Checkout means using JS code provided by Stripe to handle
almost all of the UI, which is great for us.

There are more features we should add to this page and changes we
should make, but this gives us an MVP.

[greg: expanded commit message; fixed import ordering and some types.]

zproject/settings.py

@@ -15,6 +15,7 @@ import os
 import platform
 import time
 import sys
+from typing import Optional
 import configparser
 
 from zerver.lib.db import TimeTrackingConnection
@@ -39,7 +40,7 @@ if PRODUCTION:
 else:
     secrets_file.read(os.path.join(DEPLOY_ROOT, "zproject/dev-secrets.conf"))
 
-def get_secret(key: str) -> None:
+def get_secret(key: str) -> Optional[str]:
     if secrets_file.has_option('secrets', key):
         return secrets_file.get('secrets', key)
     return None